^ ^ - THE C A U S E S O P F L U C T U A T I O N S I N TTTEGESCENCE
fihrafive escape of fltiid from the iuierior pnl-riaar tissues dependent on locaJ increments
of external prossuro there. Such increments do not, of eom-se, affect the leverage of the
cHstal portions of the leaf, but merely give rise to modifications in the relative strengths
of the opposed masses of puhanar parenchyma of such a nature as to confer increased
rektiro strengt-h upon the superior one; and hence the movement of the petiole follows
a normal course and depression results.
The secondary pidvini differ structiu-ally in many respects from the primary ones,
and much more closely resemble tire pulvini of common nyctitropic leaves than the
latter do. I'he fihro-vascular axis, hke that of the piimary pulvinus, has the form of
a solid cord devoid of any medullary content, but it is only slightly flattened vertically,
and tlierofoj-e presents no special facihties for flexion in one direction rather than another
(Plate VI, Pig. oa). The following figures show the measurements of the various portions
of two sections derived from distinct pulvini; the line of section in the first case having
fallen parallel with the length of the fibro-vascular bundle (Plate VI, Figs. 7, 8), and
in the siicond at rig'ht angles to it (Plate VI, Fig. 6a)—
Scetion 1.—Total breadth o£ pulvmis ... ... 1-005 m,m.
BreadUi of external mass of parenchyma ... .,, 0-33 „
„ fibio-vasculnr lumdle ... ... 0'33
„ asillnry mass of parenchyma .,, ,,. 0-345
Section 2-—Transverse diameter of entire puWaus ... ... 1'08 „
>• )i of outer paienoliyma ... 0-33 „
» of fibro-vascular bundle ... 0-39 „
u 1) of axillary parencliyma ... 0'36 „
TMokness of cell-walls on the outer mass of parenobyjua 0-0018 to 0'0036
„ of oell-wallsia axillary mass of parenchyma 0-0018 to 0-0027 „
Vertical diameter of entire pulvinus ... ... i -n
„ „ superior parencbyma ... .., 0 39
„ „ fibre-vaseulai bundle ... ... 0-33
» j! inferior parenebyma ... ... 0 39
These figures show that the fibro-vascular bundle is much more rounded than in the
primary puMiius, and relatively strong in relation to the surrounding parenchyma ia
every direction; and also that the pulvinus as a whole, in place of being flattened
vertically, is somewhat compressed laterally. The horizontal diameter of the fibro-vascular
buncUe sometimes, as in the second section, slightly exceeds tliat of either of the lateral
masses of ])are]ichyma and, hi other cases, falls shglitly short of that of the inner one,
us in the first section. The walls of the cells in tlie outer mass of parenchyma are somewliat
thicker than they are in the axillary one, but no sach conspicuous difference in
this re^^pect is present here as in the case of the cell-walls in the superior and inferior
parenchyma of the primary pulvmus. The tissue of the internal or axillary mass is
slightly richer in chlorophyll than that of the outer one. There is no. evidence of tiie
presence of two sti-uctm-ally distinct layers in the thickness of the parenchyma. It
darkens uniformly down to the bundle-sheath under treatment with some acid, and there
is an entire absence of any great system of intercellular spaces like that in 'the deeper
stratum in the parenchyma of the primaiy pulvinus. A certain nmnber of cells provided
mth large and elaborate pits is, however, present in the immediate neighbourhood of the
bundle-sheath; but these cells do not iorm a continuous layer, and they are irregularly
distributed. The only features, then, which distinguish the axillary from the outer mass of
I N T H E M O T O R ORGA-VS O F L E A V E S . 89
parenchyma, arc shght excess in bulk (Plate VI, Fig. 7), slight excess in the thickness of
the walls of the constituent cells, and slight excess in chlorophyll content. There are no
intrinsic stmctural features in the secondary pulvini implying such instability in equilibrum
of position as there are in the primary ones; and, in addition, so long as the primary
petiole is not deeply depressed, distal leverage does not come into play as an extrinsic
factor of disturbance, and with this we find much less Hability to the occurrence of
extensive and especially of rapid extensive movements in the secondary rachises than in
the primary petiole. Very considerable periodic movements of divergence and convergence
are regularly carried out as in other nyctitropic leaves in connection with the incidence
or removal of solar sthnulation; but there is very much less tendency to the occurrence
of extensive and rapid movements under the influence of other factors than there is in
the case of the primary petioles and pinnules—conditions wliich determine the fullest
1 of the primary petioles and maximal movements of the pinnules often leaving the
secondary rachises almost entii-ely unaffected (Plate V, Figs. 12, 13); and, if the intrinsic
structural peculiarities of the tissues of the secondai-y pulvini alone be taken into account,
this is not so much a matter for surprise as that extensive movements should occur at all.
The opposing masses of pulvinar parenchyma acting on the relatively strong fibro-vascular
coi-d differ so slightly in character from one another that, if attention be Innited to them,
it would be hard to explain the occurrence even of-regular nyctitropic movements of an
extensive character. There are no intrinsic structural pecnliari ties present in the pulvini
to justify us in supposing that any very considerable fluctuations in the relative strengths
of the axillary and external masses of pulvinai- parenchyma take place; and, in spite of
this, very extensive movements of convergence and divergence regularly succeed one
anotlier every evening and morning. When, however, the relations of the opposing masses
of parenchyma to neighbom-ing parts are taken into account, this apparent anomaly disappear.
s (Plate VI, Figs. 9, 10, 11). The base of the outer mass of parenchyma" is in
immediate contact with a mass of tissue projecting m bracket-fashion from the axis and
composed of very delicate cells poor in protoplasm and specially poor in chroinatophores.
The lower portions of the axillary masses, on the other hand, ai-e either in close contact
with the side of the racliis, which presents httle compressable tissue, or in the case of
tei-minal pulvini, -with the coixefponding mass of the neighbouring pulvinus. The bracket
or cushion on -which the base of the outer mass rests is compoi'ed of tissue which is weak
alike structurally and functionally, and consequently presents little resistance to compression,
so that, -when tui-gescence rises in the axillary parenchyma, displacement is
effected not so much by compression of the tissue of the outer parenchyma as by compression
of the bracket upon wliii-h it rests. There is probably only an inconsiderable
excess in rise in tm-gescence in the axillary mass as compared mth the outer one under
the influence of solar sti-mulation; - but the rise that does occur acts at an advantage in
effecting displacement becausc it has a resistent basis to work upon. The turgescent
tissue of the axillai-y parenchyma is supported by the resistance of the lateral sm-face of
the rachis or by the equally turgescent tissue of the neighbouring pulvinus, whereas
the external parenchyma rests on an insecure foundation, and readily sustains displacement.
From its relations to neighbouring parts, the axillary tissue, when its tm-gescence
rises under solar stimulation, is able to overcome the resistance of the relatively strong
passively convergent fibro-vascular bundle, which receives little support from the external
parenchyma owing to the insecure foundation on which the Intter rests. The struggle
.NN. Roy. Bot. Gaud. C a l c u t t a Vor- VI.